The electrospray method, laser spray method and MALDI method, etc., are typical methods of ionizing a sample. The laser spray method is described in, e.g., I. Kudaka, T. Kojima, S. Saito and K. Hiraoka “A comparative study of laser spray and electrospray”, Rapid Commun. Mass Spectrom. 14, 1558-1562 (2000). Further, the MALDI method is described in K. Dreisewerd “The Desorption Process in MALDI”, Chem. Rev. 2003, 103, 395-425.
Among these ionization methods, the laser spray method, which ionizes a liquid sample by irradiating, with a laser beam, the end of a capillary into which a liquid sample has been introduced, is advantageous in that it has a detection sensitivity that is an order of magnitude higher than that of the electrospray method. Further, whereas the existing electrospray method is difficult to apply to a sample of an aqueous solution, the laser spray method has the advantage of being applicable to samples of aqueous solutions.
The MALDI method, on the other hand, irradiates a sample, which is mixed with and held by a matrix, with a laser beam to ionize the sample. In general, use is made of an ultraviolet nitrogen laser (wavelength: 337 nm). However, as the energy density of the laser beam is high, a problem which arises is that if the sample is a biological sample, the sample will be decomposed. In the mass analysis of DNA molecules and proteins, etc., it is desired that weakly bound samples having molecular weights that exceed several tens of thousands be ionized without being caused to decompose.